Salmonella enter intestinal epithelial cells during infection of their hosts. Interestingly, bacterial invasion into host cells is not an enduring feature of Salmonella pathogenesis. The ability of Salmonella to enter non phagocytic cells appears to change in vivo. The objective of this application is to investigate the molecular mechanisms which control S. typhimurium invasiveness. These studies will lead to a better understanding of how Salmonella decide to infect non-phagocytic host cells during infection. The main goal of this project is to investigate the role of HilA in the regulation of Salmonella invasion genes and pathogenesis. HilA is a transcriptional activator that regulates the expression of many, if not all, invasion genes in S. typhimurium. Many environmental and regulatory factors control the expression of hilA and, thus, indirectly control the expression of invasion genes. The complex regulation of hilA and invasion genes appears to be an important mechanism to limit bacterial invasiveness during infection. The hilA promoter and the transcriptional regulators that directly control the hilA promoter will be characterized to understand how environmental conditions regulate hilA expression. Invasion gene promoters and the HilA protein will be examined to discern how HilA coordinately activates invasion genes. Post-translational modulation of HilA activity may provide an additional level of regulation. Studies are proposed to determine whether and how the activity of HilA might be modulated. The importance of hilA and invasion gene regulation for pathogenesis will be tested by analysis of bacterial gene expression and invasiveness during infection. Salmonella infection remains a national and worldwide health problem. Ultimately, chemotherapies might be designed to inhibit the expression of invasion genes and prevent this early step in Salmonella infection.